1. Field of the Invention
The present invention relates to a micromirror unit used in optical apparatus for the purposes of changing the direction of light. In particular, it relates to a micromirror unit of the type which is advantageously incorporated in an optical disk apparatus (for writing to or reading data from an optical disk), an optical switching apparatus (for selectively connecting one optical fiber to another to provide a light passage), etc.
2. Description of the Related Art
A micromirror unit is provided with a reflective mirror member which is pivotable for changing the direction of reflected light. A popular technique for actuating the mirror member is to utilize electrostatic force. Micromirror units of this type (referred to as xe2x80x9cstatic driving typexe2x80x9d hereinafter) may have several structures. Such micromirror units are generally classified into two groups, depending on fabrication methods. One of the methods employs a xe2x80x9csurface micro-machiningxe2x80x9d technique, whereas the other employs a xe2x80x9cbulk micro-machiningxe2x80x9d technique.
In accordance with the surface micro-machining, patterned material layers in lamination may be formed on a base substrate, thereby providing required components such as a support, a mirror member and electrodes. In this layer forming process, a dummy layer (or sacrificial layer), which will be removed later, may also be formed on the substrate. A conventional micromirror unit of the static driving type by the surface micro-machining is disclosed in JP-A-7(1995)-287177 for example.
In accordance with the bulk micro-machining, on the other hand, a base substrate itself is subjected to etching, thereby providing required components such as a frame and a mirror forming base. Then, a mirror member and electrodes may be formed on the etched substrate by a thin-film forming technique. Micromirror units of the static driving type by the bulk micro-machining are disclosed in JP-A-9(1997)-146032, JP-A-9-146034, JP-A-10(1998)-62709 and JP-A-2000-13443.
One of the technically significant factors desired in a micromirror unit is a high flatness of the reflective mirror member. According to the above-mentioned surface micro-machining technique, however, the thickness of the resulting mirror member is rendered very small, so that the mirror member is liable to warp. To avoid this and ensure a high flatness, the mirror member should be made so small that its respective edges are less than 100 xcexcm in length. In accordance with the bulk micro-machining, on the other hand, a rather thick substrate is processed, thereby providing a sufficiently rigid mirror forming base to support the mirror member. Thus, a relatively large mirror member having a high flatness can be obtained. Due to this advantage, the bulk micro-machining technique is widely used to fabricate a micromirror unit having a large mirror member whose edges are more than 100 xcexcm in length.
FIG. 10 of the accompanying drawings shows an example of conventional micromirror unit fabricated by the bulk micro-machining technique. The illustrated micromirror unit 400 is of the static driving type, and includes a lamination of a mirror substrate 410 and a base substrate 420. As shown in FIG. 11, the mirror substrate 410 includes a mirror forming base 411 and a frame 413. The mirror forming base 411 has an obverse surface upon which a mirror member 411a is formed. The mirror forming base 411 is supported by the frame 413 via a pair of torsion bars 412. The mirror forming base 411 has an reverse surface upon which a pair of electrodes 414a and 414b is formed. As shown in FIG. 10, the base substrate 420 is provided with a pair of electrodes 421a and 421b which faces the above-mentioned pair of electrodes 414a and 414b of the mirror forming base 411.
With the above arrangement, the electrodes 414a, 414b of the mirror forming base 411 may be positively charged, whereas the electrode 421a of the base substrate 420 may be negatively charged. As a result, an electrostatic force is generated between these electrodes, thereby turning the mirror forming base 411 in the N3-direction shown in FIG. 10 as the torsion bars 412 are being twisted. The rotation angle of the mirror forming base 411 is determined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars 412. To rotate the mirror forming base 411 in the opposite direction, the other electrode 421b of the substrate 420 may be negatively charged. As readily understood, when the mirror forming base 411 is turned clockwise or counterclockwise, as required, the light reflected on the mirror member 411a is directed in the desired direction.
As noted above, the mirror forming base 411 is rotated through an angle which is defined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars 412. Thus, it is possible to adjust the rotation angle of the base 411 by controlling the static electricity to be generated in correlation with the restoring force of the torsion bars 412.
Generally, a micromirror unit is a structure whose minimum dimension is about several hundred micrometers. This is rather large size, and therefore the restoring force of the torsion bars tends to exceed the inter-electrode electrostatic force in strength. Thus, conventionally, the area of each electrode is rendered large (for generating a great electrostatic force), whereas each torsion bar is made uniformly thin along its length (for weakening the restoring force). In the prior art micromirror unit 410 (FIG. 11), each torsion bar 412 has a constant small width L along the entire length.
In the above manner, however, the mirror forming base 411 is supported by the thin torsion bars 412. Accordingly, it is difficult to hold the mirror forming base 411 stable (i.e., nonrotatable) about the normal N3 (the line at right angles to the surface). If unstable about the normal N3, the mirror forming base 411 is liable to unduly swivel about the normal N3 when the base 411 is supposed to rotate only about the axis defined by the torsion bars 412. When such an unwanted swivel occurs, it is difficult or even impossible to precisely control the operation of the micromirror unit.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a micromirror unit which does not suffer from the above-noted problems. Specifically, an object of the present invention is to provide a micromirror unit which is provided with torsion bars of reduced restoring force and still can exert excellent stability against undesired swiveling.
According to a first aspect of the present invention, there is provided a micromirror unit which includes: a first frame; a mirror forming base provided with a mirror surface; and a first torsion connector which includes a first end connected to the mirror forming base and a second end connected to the first frame. The torsion connector defines a first axis about which the mirror forming base is rotated relative to the first frame. The torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to the first axis. The width of the first torsion connector is relatively great at the first end and becomes gradually smaller from the first end toward the second end.
In a preferred embodiment, a micromirror unit further includes a second frame and a second torsion connector. The second torsion connector connects the second frame to the first frame and defines a second axis about which the first frame and the mirror forming base are rotated relative to the second frame.
In another preferred embodiment, the second torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to the second axis, wherein the width of the second torsion connector is relatively great at a connecting portion to the first frame, and becomes gradually smaller from the first frame toward the second frame.
Preferably, the first torsion connector may include a plurality of torsion bars.
Preferably, a micromirror unit may further include a first potential conducting path and a second potential conducting path, wherein each of the torsion bars is connected to one of the first and the second potential conducting paths.
Preferably, the width of the first torsion connector becomes monotonically smaller from the first end to the second end.
In a preferred embodiment, the first torsion connector includes an intermediate point between the first end and the second end. The width of the first torsion connector becomes monotonically smaller from the first end to the intermediate point and becomes monotonically greater from the intermediate point to the second end.
Preferably, the first torsion connector has a rectangular cross section or a circular cross section or an elliptical cross section.
Preferably, the first torsion connector has a hollow structure.
Preferably, the first torsion connector includes a bifurcating portion.
Preferably, the first torsion connector may include, in at least one of the first end and the second end, a curved portion for prevention of stress concentration.
In a preferred embodiment, the mirror forming base is provided with a first comb-teeth electrode, while the first frame is provided with a second comb-teeth electrode cooperating with the first comb-teeth electrode for moving the mirror forming base.
Preferably, a micromirror unit may further include a support base facing the mirror forming base. The support base is provided with a first electrode facing the mirror forming base, while the mirror forming base is provided with a second electrode facing the first electrode.
Preferably, the mirror forming base may be provided with a first electromagnetic coil, and the support base may be provided with a second electromagnetic coil or a permanent magnet facing the first electromagnetic coil.
Preferably, the mirror forming base may be provided with a permanent magnet, and the support base may be provided with an electromagnetic coil facing the permanent magnet.
Preferably, at least a part of the first frame may have a multi-layer structure including a plurality of conductive layers and an insulating layer disposed between the conductive layers.
Preferably, the first frame may be provided with a third comb-teeth electrode, and the second frame may be provided with a fourth comb-teeth electrode cooperating with the third comb-teeth electrode for moving the first frame and the mirror forming base.
According to a second aspect of the present invention, there is provided a micromirror unit which includes: an inner frame; an outer frame; a mirror forming base provided with a mirror surface; an inner torsion connector connecting the inner frame to the mirror forming base; and an outer torsion connector which connects the inner frame to the outer frame and defines an axis about which the inner frame and the mirror forming base are rotated relative to the outer frame. The outer torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to said axis. The width of the outer torsion connector is relatively great at a connecting portion to the inner frame, and becomes gradually smaller from the inner frame and to the outer frame.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.